METHOD AND DEVICE FOR REMOVING REACTIVE PARTICLES FROM A VACUUM ENVIRONMENT, PROCESS PLANT FOR PRODUCING MONOCRYSTALLINE SILICON INGOTS

Description

[0001] The invention relates to a method and a device for removing reactive particles from a vacuum environment. The invention also relates to a process plant for producing monocrystalline silicon ingots.

[0002] The need to remove reactive particles from a vacuum environment often arises in process plants from which process gases are extracted. One example is the production of monocrystalline silicon ingots formed from a silicon melt. The silicon melt is contained within a vacuum chamber into which argon is introduced as a purge gas. During the silicon ingot formation process, a continuous flow of process gas is maintained by continuously introducing argon into the chamber and extracting it from the chamber using a vacuum pump.

[0003] The process gas comes into contact with the silicon ingot, the silicon melt and other surfaces in the vacuum housing, and therefore carries reactive particles with it when exiting the vacuum housing.

[0004] Up to now, it has been common practice to accumulate the reactive particles in a filter located between the vacuum housing and the vacuum pump until the formation of the silicon ingot is complete. A phase in which the silicon ingot is removed from the vacuum housing or in which the silicon melt is renewed can be used to clean the filter and remove the reactive particles.

[0005] If an excessive amount of reactive particles accumulates in a filter, problems can arise when cleaning the filter. For example, dust pockets can form that cannot be removed by simply blowing it out. Furthermore, chemical reactions can be triggered by contact of the reactive particles with oxygen, releasing heat energy. With a large quantity of reactive particles, the amount of heat released can be so great that the filter is damaged. WO 2007 / 076867 A1, DE 10 2011 050247 A1, WO 2015 / 179884 A2, US 2018 / 290093 A1, DE 195 16 925 A1 and DE 10 2005 022101 A1 are relevant prior art documents.

[0006] The invention is based on the objective of presenting a method and a device for removing reactive particles from a vacuum environment, as well as an associated process plant, with which these disadvantages are avoided. This objective is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0007] The invention relates to a method for removing reactive particles from a vacuum environment, in which a process gas is drawn from the vacuum environment by a vacuum pump and is passed between the vacuum environment and the vacuum pump through a first filter and a second filter to filter out reactive particles. A liquid ring pump removes the particles from the first and second filters. In a first phase of the method, the first filter is active and the second filter is passive. In a second phase of the method, the first filter is passive and the second filter is active. In the first phase, process gas is passed through the first filter and the liquid ring pump removes particles from the second filter. In the second phase, process gas is passed through the second filter and the liquid ring pump removes particles from the first filter.

[0008] By alternately using two filters connected in parallel between the vacuum environment and the vacuum pump, it becomes possible to determine the timing for filter cleaning independently of the processes within the vacuum housing. While one filter is active and filters reactive particles from the process gas pumped by the vacuum pump, the other filter can be switched to a passive state, in which it does not contribute to the processes within the vacuum housing. In the passive state, the reactive particles can be removed from the filter using the liquid ring pump. This alternating operation, in which one filter is active and the other passive, allows for a continuous supply of process gas from the vacuum environment without excessive accumulation of reactive particles in either filter.

[0009] The process is preferably carried out such that the process gas is continuously drawn from the vacuum environment between the first and second phases. There can be one or more transition phases between the first and second phases of the process. The transition phase can include a section in which the process gas is passed through both the first and second filters simultaneously. The time required to remove the particles from the passive filter may be shorter than the time required to filter the process gas with the active filter. The transition phase can then include sections in which the passive filter is completely inactive, i.e., it neither contributes to filtering the process gas nor is cleaned.

[0010] The first filter can be designed to filter reactive particles from the process gas by passing the process gas through a filter material on which the reactive particles condense. The filter material can be, in particular, a porous material. The filter material can separate a primary filter chamber from a secondary filter chamber. The process gas, coming from the vacuum environment, can be introduced into the primary filter chamber and pass through the porous material into the secondary filter chamber. The filtered process gas can then be discharged from the secondary filter chamber using a vacuum pump.

[0011] The porous material can form a tubular structure. The outer surface of the tubular structure can adjoin the primary filter chamber, while the inner surface can adjoin the secondary filter chamber. The tubular structure can be oriented vertically. One end of the tubular structure can be closed, and the other end can be open. The open end can connect to the secondary filter chamber. The open end can also be the top of the tubular structure. The secondary filter can have the same characteristics as the primary filter.

[0012] While the process gas is being filtered by the active filter, a vacuum is maintained within the active filter. After switching to the passive state, an oxygen-containing gas can be introduced into the passive filter during an initial transition phase, thus increasing the pressure within the passive filter. In one embodiment, ambient air is introduced into the passive filter, bringing it to atmospheric pressure. It is also possible to introduce compressed air into the passive filter at a pressure higher than atmospheric pressure, or a gas with a higher oxygen content than that of air. For this purpose, the filter can include a vent valve that opens to introduce the gas and is closed when the filter is in its active state. The gas can be introduced at the beginning of the passive state of the filter, i.e., before the particles are removed from the filter by the liquid ring pump.

[0013] The introduction of the gas leads to chemical reactions, particularly between the reactive particles and oxygen, which release heat energy. The reactive particles lose at least some of their reactivity, thus reducing the risk of ignition or explosion.

[0014] The oxygen-containing gas, which floods the interior of the passive filter, can be introduced into the secondary filter chamber. This creates a countercurrent flow through the filter material, running in the opposite direction to the flow of the process gas in the active state of the filter. The countercurrent can extend from the secondary filter chamber, through the filter material, and into the primary filter chamber. The vent valve can be opened quickly, resulting in a sudden inflow into the interior of the passive filter. This countercurrent dislodges particles that have settled on the filter material. Simultaneously, the reactive particles can react due to the intense contact with the incoming gas, thus reducing their reactivity. The particles detached from the porous material initially disperse within the primary filter chamber and then settle to the bottom.

[0015] The liquid ring pump can be connected to a lower section of the first and / or second filter, allowing accumulated particles to be removed. Alternatively, the pump can be connected to the primary filter chamber of the first and / or second filter. The bottom of the first and second filters can be sloped to direct the particles towards the pump connection. The pump connection can be located at the bottom of the slope. While the particles are being removed from the passive filter, free air exchange can occur between the interior of the passive filter and the surrounding environment, thus preventing a vacuum from forming inside the filter.

[0016] The removal of particles from the passive filter can be facilitated if the particles accumulated at the bottom of the passive filter are fluidized. A fluidizing device can be provided to introduce a fluidizing gas stream into the accumulated particles.

[0017] The particles discharged towards the liquid ring pump mix inside the pump with the operating fluid that forms the liquid ring. Chemical reactions between the particles and the operating fluid can further reduce the particles' reactivity. The particles can be discharged from the liquid ring pump along with the pump's operating fluid. The operating fluid can be exchanged during operation, so that fluid with a higher particle count is discharged from the pump and fluid with a lower particle count is supplied to it.

[0018] The operating fluid can be continuously replaced during operation of the liquid ring pump. The operating fluid can be water. Fresh or treated water can be supplied to the liquid ring pump. It is also possible to accumulate the particles in the operating fluid up to a predetermined concentration and replace the operating fluid when this concentration is reached.

[0019] The feature of a liquid ring pump according to the invention does not imply any limitation regarding the number of components. The liquid ring pump according to the invention can consist of two components, such that the first component is connected to the first filter and the second component is connected to a second filter. Preferably, the liquid ring pump is designed as a single component that communicates alternately with the first filter and the second filter.

[0020] The gas pumped by the liquid ring pump can be collected or vented to the environment. The gas may contain reactive gaseous components such as hydrogen. To avoid hazards, the gas can be diluted after exiting the liquid ring pump, for example by adding air, until the concentration of flammable substances in the gas is below the lower explosive limit (LEL).

[0021] Following particle removal, the passive filter can be prepared for active operation in a second transition phase. This involves closing the connection between the passive filter's interior and the liquid ring pump. The vent valve, which allows the passive filter's interior to communicate with the environment, can also be closed. A vacuum can then be applied inside the passive filter, matching the vacuum level of the active filter. Once both filters are at the same pressure, the passive filter can be connected to the process gas flow between the vacuum housing and the vacuum pump. After this connection is established, the active filter can be disconnected from the process gas flow, thus transitioning to the passive state.

[0022] The cleaning interval, i.e., the duration a filter operates in the active state, can be determined based on the condition of the active filter. One criterion, for example, could be that the pressure difference between the primary and secondary filter chambers has exceeded a predefined threshold. A high pressure difference can indicate that a certain amount of reactive particles has accumulated in the filter. Additionally or alternatively, the weight of the active filter can be used to estimate the amount of reactive particles present, and the filter can be switched to the passive state when a predefined weight threshold is exceeded. In another variant, the filter switches from the active to the passive state after a predetermined time period.

[0023] Evacuating the passive filter before it transitions to the active state can be done with the same vacuum pump that also generates the vacuum for the surrounding environment. This approach can negatively affect process stability in the vacuum environment because pressure fluctuations can occur during the evacuation of the passive filter. Therefore, one embodiment provides an auxiliary vacuum pump to evacuate the passive filter before it transitions to the active state.

[0024] The auxiliary vacuum pump can also evacuate an airlock chamber through which objects are introduced into or removed from the vacuum environment. Before objects pass between the vacuum environment and the airlock, the airlock chamber is evacuated to the same pressure as the vacuum environment. This is also the pressure to which the passive filter is brought before it switches to its active state, so the same requirements apply to the auxiliary vacuum pump in both cases.

[0025] The process gas conveyed by the (main) vacuum pump can be fed to a conditioning station where it is treated to make it suitable for reuse in the process within the vacuum environment. In particular, argon can be recovered from the process gas in the conditioning station and made available for reuse. A connecting line can exist between the conditioning station and the vacuum environment, allowing the treated process gas to be returned to the vacuum environment in a closed loop.

[0026] The feature of a vacuum pump according to the invention does not include any limitation regarding the number of components. The vacuum pump according to the invention can consist of two components, such that the first component is connected to the first filter and the second component to the second filter. Preferably, the vacuum pump is designed as a single component that communicates alternately with the first filter and the second filter.

[0027] In one embodiment, the vacuum pump according to the invention is designed as a sequence of two vacuum pump units connected in series. The input of the second vacuum pump unit can be connected to the output of the first vacuum pump unit, so that only a portion of the pressure difference between the vacuum environment and atmospheric pressure is applied across each of the vacuum pump units. In this way, the energy efficiency of the vacuum pump can be improved.

[0028] To prevent the process gas from being contaminated by operating fluids or lubricants of the vacuum pump, the vacuum pump is preferably designed as a dry-running vacuum pump. In a preferred embodiment, the vacuum pump is a screw pump. The same can apply to the auxiliary vacuum pump and / or the vacuum pump assemblies.

[0029] The invention also relates to a device for removing reactive particles from a vacuum environment, comprising a vacuum housing and a vacuum pump connected to the vacuum housing. A first filter and a second filter are arranged between the vacuum housing and the vacuum pump to filter reactive particles from a process gas conveyed by the vacuum pump. The device includes a liquid ring pump for extracting particles from the first and second filters. A switching device brings the device into a first switching state and a second switching state, such that in the first switching state the process gas is passed through the first filter and the liquid ring pump removes particles from the second filter, and in the second switching state the process gas is passed through the second filter and the liquid ring pump extracts particles from the first filter.

[0030] The invention further relates to a process plant comprising a vacuum housing and a device according to the invention connected to the vacuum housing for removing reactive particles from the vacuum environment of the vacuum housing. The process plant may include an airlock chamber for introducing objects into and / or removing them from the vacuum housing. The process plant may include an auxiliary vacuum pump designed to evacuate the airlock chamber and to evacuate the passive filter before it transitions to the active state. The process plant may include a closed process gas circuit extending from the vacuum housing via the vacuum pump to a process gas conditioning station and from the process gas conditioning station back to the vacuum housing.

[0031] The process plant can be designed to produce monocrystalline silicon ingots. A melting furnace can be arranged in the vacuum housing for generating molten silicon. The transfer chamber can be designed to introduce a silicon core into the vacuum housing and to eject the finished silicon ingot from the vacuum housing. The invention also relates to a method for operating such a process plant.

[0032] The disclosure includes further developments of the device and the process plant that are described in connection with the method according to the invention. The disclosure also includes further developments of the method that are described in connection with the device or process plant according to the invention.

[0033] The invention is described below by way of example with reference to the accompanying drawings and advantageous embodiments. The drawings show: Fig. 1 : a first embodiment of a process plant according to the invention; Fig. 2 : a filter from Fig. 1 in an enlarged view; Fig. 3 : a second embodiment of a process plant according to the invention.

[0034] One in Fig. 1 The process plant shown comprises a vacuum housing 14 in which a vacuum is created by a system consisting of a first screw pump 21 and a second screw pump 22. The system of screw pumps 21 and 22 forms a vacuum pump according to the invention. The screw pump 21 receives information about the pressure in the vacuum housing 14 from a first pressure sensor 41, so that a predetermined pressure can be generated in the vacuum housing 14 during controlled operation. A crucible 16 made of a ceramic material, which is open at the top, is arranged in the vacuum housing 14. The crucible 16 is surrounded by a heating device 17, so that a silicone melt 18 can be provided in the crucible 16.

[0035] The process plant comprises a lock chamber 19 into which a silicon seed crystal is introduced at atmospheric pressure. After the lock chamber 19 is closed, a vacuum is created in the lock chamber 19 using a third vacuum pump 20. The third vacuum pump 20, which constitutes an auxiliary vacuum pump according to the invention, receives information about the pressure in the lock chamber 19 from a second pressure sensor 38, so that a predetermined pressure can be generated in the lock chamber 19 during controlled operation. As soon as the pressure in the lock chamber 19 matches the pressure in the vacuum housing 14, the lock chamber 19 is opened to the vacuum housing 14. The seed crystal is lowered on a wire rope until it comes into contact with the surface of the melt 18. As the wire rope is slowly withdrawn, silicon material from the melt 18 is deposited onto the seed crystal, forming a silicon ingot 15.The finished silicon ingot is transferred to the airlock chamber 19. The airlock chamber 19 is separated from the vacuum housing 14, the valve 40 is closed, and the airlock chamber 19 is brought back to atmospheric pressure so that the silicon ingot 15 can be removed.

[0036] Argon is continuously introduced into the vacuum housing 14 from an argon supply 39 as a process gas. The argon acts as a purge gas, removing interfering particles and other atmospheric components from the vacuum housing 14. These interfering particles are formed, for example, in the form of silicon oxides when reactions occur between the melt and oxygen. The presence of oxygen in the vacuum atmosphere cannot be completely prevented, for example, due to outgassing from components within the vacuum housing 14.

[0037] The melt may contain materials for doping the silicon ingot. In the case of N-doped single crystals, for example, red phosphorus is a suitable doping material. However, highly reactive dusts can form from red phosphorus, which can interfere with the formation of the silicon ingot.

[0038] The reactive particles are captured by the flow of argon purge gas, which is maintained by the screw pumps 21, 22, and discharged from the vacuum housing 14. With the device according to the invention, the enriched argon purge gas is freed from the reactive particles before it reaches the first screw pump 21.

[0039] For this purpose, a first filter 31 and a second filter 32 are arranged between the vacuum housing 14 and the first screw pump 21. The filters 31 and 32 are parallel to each other, so that the process gas can pass through either the first filter 31 or the second filter 32. This allows one of the two filters 31 or 32 to be put into a passive state in which it can be cleaned. A control unit 57 actuates valves 23, 24, 25, 26, 27, 28, 29, 30, 33, 34, and 44, so that each valve assumes the desired state. The control unit 57 forms a switching device according to the invention.

[0040] In the first phase of an operating cycle, the first filter 31 is active and the second filter 32 is passive. Valves 27 and 37 are open, while valves 28, 30, 44, and 33 are closed, allowing the process gas from the vacuum chamber 14 to flow through the first filter 31 to the first screw pump 21. Valves 25 and 26 are closed, preventing process gas from flowing through the second filter 32.

[0041] The first filter 31 has a stainless steel housing containing a partition 49 that separates a primary filter chamber 47 from a secondary filter chamber 48. The primary filter chamber 47 has an inlet opening 45 that communicates with the vacuum housing 14. The secondary filter chamber 48 has an outlet opening 46 that communicates with the first screw pump 21. Between the primary filter chamber 47 and the secondary filter chamber 48, the process gas passes through a filter cartridge 52 made of a porous material. The reactive particles contained in the enriched process gas are deposited on the outside of the filter cartridge 52 and in the pores, so that the process gas entering the interior of the filter cartridge 52 is free of the reactive particles. The purified process gas exits the first filter 31 via the secondary filter chamber 48 and is directed to the first screw pump 21.Since the process gas is free of reactive components, it can be safely released into the environment at the outlet of the second screw pump 22. Over time, increasing amounts of particles accumulate on the filter cartridge 52, necessitating regular cleaning of the filter to remove these particles.

[0042] The second filter 32 is constructed identically to the first filter 31. After a phase in the active state, the second filter 32 is switched to the passive state to perform the cleaning process. After closing the valves 25 and 26, there is no longer a flow of process gas between the inlet opening 45 and the outlet opening 46. As a first step, the vent valve 24 is opened, allowing air from the atmosphere to enter the secondary filter chamber 48 through an air inlet 50. Alternatively, a compressed air source or an oxygen supply can also be connected to the vent valve 24. Due to the pressure difference between atmospheric pressure and the pressure inside the second filter 32, a strong flow occurs from the secondary filter chamber 48 into the primary filter chamber 47, which passes through the filter cartridge 52 in a counter-current flow.Particles adhering to the filter candle 52 are detached and initially distributed with the airflow in the primary filter chamber 47 before sinking to the bottom.

[0043] Upon contact with atmospheric oxygen, the particles react, releasing heat. The cleaning cycles are scheduled so that the released heat is insufficient to damage the second filter 32. A grid 55 is arranged parallel to the bottom of the second filter 32, which is designed as an inclined surface 53. An airflow connected to the valve 27 is introduced into the second filter 32 through a fluidizing opening 54. This airflow spreads between the inclined surface 53 and the grid 55 and passes through the grid from below. The airflow fluidizes the particles that collect at the bottom of the second filter 32.

[0044] The fluidized particles are drawn out of the second filter 32 through a cleaning opening 51 by a liquid ring pump 35. The phase in which the liquid ring pump 35 is in operation to remove particles from the second filter 32 is the first phase of an operating cycle according to the invention. A preceding phase is referred to as the first transition phase, and a phase following the first phase is referred to as the second transition phase.

[0045] During operation, the liquid ring pump 35 is continuously supplied with fresh water as the operating fluid. A corresponding quantity of operating fluid is discharged via the outlet of the liquid ring pump 35 and conveyed to a collection tank 36. The particles discharged from the second filter 32 mix with the operating fluid and enter the collection tank 36 along with it. Particles that have not yet completely lost their reactivity can react further upon contact with the operating fluid. Gaseous components are discharged upwards from the collection tank 36. If reactive gaseous components are present, the discharged gas can be diluted with air before being released into the environment. The operating fluid enriched with particles is also drawn from the collection tank 36 and sent for treatment.

[0046] After the particles have been removed from the second filter 32, valves 24, 27, and 34 are closed again, and the second filter 32 is prepared for the transition to the active state in a second transition phase. For this purpose, valve 23 is first opened, allowing the interior of the second filter 32 to be evacuated by the third vacuum pump 20. As soon as the pressure inside the second filter 32 matches the pressure inside the first filter 31, the second filter 32 is ready for the transition to the active state, and valve 23 is closed again.

[0047] A pressure sensor monitors the pressure difference between the inlet opening 45 and the outlet opening 46 of the first filter 31. The more particles accumulate in the first filter 31, the greater the pressure difference becomes, allowing a pressure difference threshold to be set. Upon reaching this threshold, the first filter 31 requires cleaning. When the threshold is reached, valves 25 and 26 open, switching the second filter 32 to the active state. Valves 29 and 37 then close, returning the first filter 31 to the passive state.

[0048] The cleaning of the first filter 31 begins as described by opening valve 30, allowing ambient air to enter the secondary filter chamber 48. After activating a fluidizing device connected to valve 28 and opening valve 44, the first filter 31 is connected to the liquid ring pump 35, allowing the particles to be extracted. Subsequently, valves 28, 30, and 44 are closed, and valve 33 is opened to evacuate the interior of the first filter 31, preparing it for reactivation. The valves are actuated in a dampened manner to minimize the impact on the process gas pressure. The exception to this is the vent valves 24 and 30, which are opened rapidly to create a sudden flow into the interior of filters 31 and 32.

[0049] In the alternative embodiment according to Fig. 3The vacuum pump used to generate the vacuum in the vacuum housing 14 is a single screw pump 21. A processing device 43 is connected to the outlet of the screw pump 21, in which the process gas, extracted from the vacuum housing 14 and freed from reactive particles, is processed. The pure argon produced by the processing is returned to the vacuum housing 14 and can be used again as a purge gas. Other components of the process gas are released into the environment.

[0050] The cleaning intervals are not determined based on the differential pressure across filters 31 and 32, but rather by time. If the process gas has passed through the active filter for a predetermined period, it is assumed that a sufficient quantity of particles has accumulated, necessitating cleaning.

Claims

1. Method for removing reactive particles from a vacuum environment (14), the method comprising conveying a process gas from the vacuum environment (14) by means of a vacuum pump (21, 22), conducting the process gas between the vacuum environment (14) and the vacuum pump (21, 22) through a first filter (31) and a second filter (32) in order to filter reactive particles out of the process gas, and abstracting particles from the first filter (31) and the second filter (32) by means of a liquid-ring pump (35), wherein in a first phase of the method the first filter (31) is active and the second filter (32) is passive, wherein in a second phase of the method the first filter (31) is passive and the second filter (32) is active, wherein in the first phase the process gas is conducted through the first filter (31) and the liquid-ring pump (35) abstracts particles from the second filter (32), and wherein in the second phase the process gas is conducted through the second filter (32) and the liquid-ring pump (35) abstracts particles from the first filter (31).

2. Method according to Claim 1, wherein process gas is continuously conveyed from the vacuum environment (14) between the first phase and the second phase.

3. Method according to Claim 1 or 2, wherein the first filter (31) and / or the second filter (32) comprise a primary filter space (47) and a secondary filter space (48) which are separated from one another by a filter material (52), and wherein the process gas crosses from the primary filter space (47) through the filter material (52) into the secondary filter space (48).

4. Method according to any of Claims 1 to 3, wherein in a first transitional phase an oxygen-containing gas is admitted into the passive filter (31, 32).

5. Method according to Claim 4, wherein the oxygen-containing gas is admitted into the secondary filter space (48), such that a counterflow through the filter material (25) is produced.

6. Method according to Claim 4 or 5, wherein the liquid-ring pump (35) is connected to the primary filter space (47) of the first filter (32) and / or the second filter (32).

7. Method according to any of Claims 1 to 6, wherein the particles abstracted from the passive filter (31, 32) are discharged from the liquid-ring pump together with an operating liquid of the liquid-ring pump (35).

8. Method according to any of Claims 1 to 7, wherein the process gas issuing from the vacuum pump (21, 22) is treated for reuse in the vacuum environment (14).

9. Method according to Claim 8, wherein argon is regenerated from the process gas.

10. Apparatus for removing reactive particles from a vacuum environment, comprising a vacuum housing (14) and comprising a vacuum pump (21, 22) connected to the vacuum housing (14), there being disposed between the vacuum housing (14) and the vacuum pump (21, 22) a first filter (31) and a second filter (32) for filtering reactive particles out of a process gas conveyed by means of the vacuum pump (21, 22), comprising a liquid-ring pump (35) for suctioning particles from the first filter (31) and the second filter (32), and comprising a switching device (57) for bringing the apparatus to a first switching state and a second switching state, such that in the first switching state the process gas is conducted through the first filter (31) and the liquid-ring pump (35) abstracts particles from the second filter (32) and such that in the second switching state the process gas is conducted through the second filter (32) and the liquid-ring pump (35) suctions particles from the first filter (31).

11. Process plant comprising an apparatus according to Claim 10, wherein monocrystalline silicon ingots are produced in the vacuum housing (14) and wherein process gas released by the vacuum pump (21, 22) is treated and is returned to the vacuum housing (14).